Conventional lithographic printing plates consist of a suitable substrate material having a surface comprised of a pattern of ink-receptive material surrounded by ink-rejective material.
In one lithographic plate, a substrate, such as an aluminum sheet is coated with a water-soluble resinous material which may be rendered insoluble and adherent to the substrate on exposure to ultraviolet light. Such a coated substrate is then covered with a mask such as a photographic negative and exposed to a source of ultraviolet light which causes the exposed portions of the coating to become insoluble and adherent to the substrate. Unexposed portions of the coating remain soluble so that subsequent washing of the plate removes all such unexposed material leaving bare substrate. In the printing process, the exposed and hardened portions of the coating are preferentially ink-receptive and form the image areas of the print. Substrate portions of the plate are preferentially water-receptive ink-rejective upon treatment and form the white areas of the print.
Common methods of exposure to ultraviolet light include the use of a lamp such as a mercury vapor lamp, a zenon lamp, a carbon arc lamp, or a fluorescent lamp. (U.S. Pat. No. 4,301,230) Alternatively, an ultraviolet laser may be employed. (U.S. Pat. No. 3,644,737)
If a laser of sufficiently narrow beam width and sufficiently high power is employed, the laser may be scanned over the surface at a rapid rate or may be modulated during such a scan thus eliminating the requirement of a differentially transparent mask.
Others in the art have employed a beam of laser radiation in order to transfer material from a transparent cover sheet onto a suitable substrate. (U.S. Pat. Nos. 3,945,381; 3,964,389; 4,245,003)
Also found in the prior art is a system comprising a photohardenable element disposed between a cover sheet and a support substrate. The cover sheet is formed of a transparent material to permit selected exposure of the underlying photohardenable compounds. (U.S. Pat. No. 4,126,466)
Despite such diverse methods of lithographic plate production in the prior art, applicants herein have found no suggestion of a thermally convertible selectively ink-receptive composition which may be suitable for direct use in a lithographic process.